1. Field of the Invention
The invention relates to a magnetic-inductive flowmeter for measuring the flow rate of a flowing conductive medium with a measuring tube which consists of a nonconductive material, with a magnetic field generating apparatus for generating an especially changing magnetic field which permeates the measuring tube essentially perpendicular to the longitudinal axis of the measuring tube, with two measuring electrodes which are especially conductively coupled to the medium for tapping a measuring-circuit voltage which has been induced in the flowing medium and with a measuring device for detecting an empty tube, the measuring electrodes being located along a connecting line which runs perpendicular to the longitudinal axis of the measuring tube and perpendicular to the direction of the magnetic field.
2. Description of Related Art
Magnetic-inductive flowmeters whose manner of operation is based on the principle of electromagnetic induction (=Faraday induction) have been known for many years and are extensively used in industrial measurement engineering. According to Faraday's Law, in a flowing medium which entrains charge carriers and which flows through a magnetic field, an electrical field intensity arises perpendicular to the flow direction and perpendicular to the magnetic field. Faraday's Law is used in magnetic-inductive flowmeters in that, by means of a magnetic field generating apparatus which conventionally has two energized magnet coils, a magnetic field is generated which is routed at least partially through the measuring tube, the generated magnetic field having at least one component which runs perpendicular to the flow direction. Within the magnetic field each volumetric element of the flowing medium, which element is moving through the magnetic field and which has a certain number of charge carriers, with the field intensity which arises in this volumetric element contributes to a measuring-circuit voltage which can be tapped via the electrodes.
Since the induced voltage which has been tapped via the electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, the volumetric flow can be directly determined from the measured voltage at a known diameter of the measuring tube. The prerequisite for the use of a magnetic-inductive flowmeter is simply a minimum conductivity of the medium. Moreover, it must be ensured that the measuring tube is filled with the medium at least to the extent that the level of the medium is above the measuring electrodes. But since when the measuring tubes are not completely filled, depending on the degree of filling, a considerable measurement error can occur, and magnetic-inductive flowmeters are optimally suitable primarily for applications in which the measuring tube is completely filled. For this reason, magnetic-inductive flowmeters, in practice, generally have a measuring device for detecting an empty tube, which indicates to the user when the degree of filling has dropped so far that the determined measured value can no longer be ascertained with the required accuracy. This can be the case, for example, in a measuring tube which is only two thirds filled so that the measuring devices used in practice for “detecting” an empty tube generate a signal not only when the measuring tube is in fact “empty”.
If a magnetic-inductive flowmeter is also to display a measured value when it is not completely filled, the degree of filling must be known so that the measured value can be corrected. These correction values and their determination are discussed for example, in German Patent DE 196 37 716 C1. For this purpose, a test signal voltage is applied to a first pair of electrodes which are opposite one another and a reaction voltage is measured which is caused, in this way, on a second pair of electrodes which are opposite one another, the two electrode pairs being electrically coupled to the medium. Based on the determined ratio between the reaction voltage and the test signal voltage, a correction value is determined, corresponding correction values having been determined empirically beforehand and having been stored in a memory.
The amount of filling of the measuring tube can also be capacitively determined according to German Patent DE 196 55 107 C2 and corresponding U.S. Pat. No. 6,092,428. Here, the two electrodes for measuring the flow rate of the medium are capacitively coupled to the medium, the electrodes with the medium as a dielectric forming a capacitor. Using the two electrodes, on the one hand, the voltage which has been induced in the medium is tapped as a measure for the flow rate of the medium, and on the other hand, the electrodes are exposed to an ac voltage and the capacitance between the electrodes is determined by a control and evaluation circuit as a measure for the proportion of the conductive medium in the measuring tube.
According to German Patent Application DE 10 2009 045 274 A1 a measurement is also enabled for a partially filled measuring tube by a magnetic field generating apparatus being arranged such that the magnetic field which has been generated by it horizontally permeates the measuring tube and there is one electrode in the bottom and the other electrode is located in the top of the measuring tube. The electrode located above the measuring tube has an electrically conductive nonmagnetic extension which is located in the measuring tube and in measurement operation is connected to the medium.
Finally, German Patent Application DE 10 2010 001 993 A1 discloses a magnetic-inductive flowmeter in which, in addition to the magnetic-inductive measuring unit, there is another temperature measuring device which has a metal housing which is in contact with the medium and which is used as an electrode for measuring a minimum conductivity. Depending on the presence of the minimum conductivity, a certain degree of filling is then deduced by an evaluation unit.
In the prior art, in part different measurement principles are combined with one another in order to obtain information about the flow rate and the liquid level. To some extent, the values for the flow rate and liquid level are also determined at different instants. In addition to the problem that the measurement of the flow rate can be influenced by the measurement or the determination of the liquid level, especially when the measuring electrodes which are intended primarily for the flow rate measurement are also included in the measurement, there is also the problem that, for the different measurement principles, the measured value for the liquid level depends on the conductivity of the medium so that it must be known in order to be able to draw a conclusion about the liquid level. Additionally, the measurement of the liquid level can be adversely affected by thin liquid films or conductive adhesions in the region of the measuring device so that it cannot be reliably distinguished whether it is “only” adhesion of the medium or there is a corresponding, relatively high liquid level.